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This would be inherently problematic with reflections/refractions and radiosity.

Maybe a better alternative would be to introduce a new image projection type, equivalent of replacing the camera with a projector, which could then easily be used with a sky_sphere (or even arbitrary geometry, which could provide tremendous potential for rendering stuff into existing images).

what seems behind the initial query would be to automatically compose an existing image as being behind the rendered scene (whenever alpha reach 100%)Only side effect would that the rendered image has coordinates in <0,0>/<1,1> and that should match the space of the background image (so it could get stretched if ratio are different)

For reflections/refraction & radiosity, I guess that out of view value is irrelevant, and as such the image could be a "replacement" (or rather a bonus) to the actual background colour.In fact, the texture is irrelevant, only the pixel image is important (with a possible gamma correction at reading, and same anti-aliasing).Finish is irrelevant. Normal is irrelevant. A simple pigment_map would be enough (without transform)

A projection type would failed with normal-perturbed camera.

Now, what about, for the original querier, to use a no_reflection, no_shadow, ambient 1 diffuse 0, flat box orthogonal to the camera (MEEEPPP, fails on normal-perturbation of camera) in the very back of the scene, with image on it ?

Or use the omni-present imagemagick as post-process command of povray to perform the composition.

Only real need would be to avoid ruling of IRTC... do we need to turn povray into gimp for such purpose ?

If done as a bonus to current background colour, it could be "easy" to do it in the file-saving/transformation, when all data has been generated.

On second thought, if the intend of the Original Poster is to specify an image as the background, it's only a matter of using a 2D image composing software afterhand, rendering with +UA (and in a format that support alpha: +FN, +FT, +FC).(well, seems to have already been my first thought too).